It is generally known for a silver-containing catalyst to be employed in the preparation of ethylene oxide from ethylene. See for example U.S. Pat. No. 3,962,136, issued June 8, 1976, and also the literature cited therein. In order to obtain improved silver catalysts, efforts have been directed for many years towards modifying the silver catalysts with the aid of promoters. For example, the above-mentioned U.S. Pat. No. 3,962,136 describes a process in which a silver compound is applied to a carrier, after which the applied silver compound is reduced to silver and in which additionally a promoter in the form of potassium oxide, rubidium oxide or cesium oxide or a mixture thereof is present on the carrier. Commercially available silver catalysts are known under the brand names of Shell S809, S829 and S839.
Treating an aluminum oxide with silicon compounds in order to improve the mechanical and thermal properties of the aluminum oxide is known from U.S. Pat. Nos. 4,013,589 and 4,013,590. These do not relate to the preparation of alpha-aluminum oxide. U.S. Pat. No. 4,392,988 mentions the impregnation of activated aluminum oxide in a silicon liquid, the separation of the aluminum oxide and its heating to a temperature of between 350.degree. C. and 1100.degree. C. No alpha-aluminum oxide is formed in this process. U.S. Pat. No. 4,379,134 refers to the use of silicon oxide and alkali metal oxides as fluxes, but this use is immediately rejected, since these fluxes could introduce undesirable impurities into the aluminum oxide. It is surprising that when employing carriers modified with the aid of alkali metals and silicon, the silver catalysts exhibit an enhanced selectivity when employed in the preparation of ethylene oxide.